This invention relates to preparation and isolation of essentially pure cyclopolydiorganosiloxane species. More specifically, this invention relates to the preparation of the cyclopolydiorganosiloxanes from the vapor-phase rearrangement of other cyclopolydiorganosiloxanes or mixtures thereof.
For the purposes of the instant invention, "cyclopolydiorganosiloxanes" or "D.sub.c " are organosiloxanes having the formula. EQU (R.sub.2 SiO).sub.c,
wherein each R is an alkyl, alkenyl, aryl, or alkaryl group, and c has a value of 3 or greater.
Cyclopolydiorganosiloxanes such as cyclooctamethyltetrasiloxane, [(CH.sub.3).sub.2 SiO].sub.4, and mixtures thereof are critical intermediates in the silicone industry, primarily as starting monomers for polymerization. As the technology of the silicones industry has advanced, the need for the individual species of cyclopolydiorganosiloxanes has become a matter of commercial interest. As an example, the cyclotrisiloxanes, D.sub.3, are of commercial interest because of their high reactivity in the formation of polymers.
Several general routes are known for the preparation of cyclopolydiorganosiloxanes. A basic process is the separation and isolation of D.sub.c from the product of hydrolysis of the corresponding diorganodihalosilane. The hydrolysis product is a mixture of D.sub.c and hydroxy endblocked linear polydiorganosiloxanes. The predominant species in the mixture of D.sub.c is the tetramer or D.sub.4. One has limited flexibility in altering the mix of species within the D.sub.c mixture. To balance the need for D.sub.c beyond D.sub.4 or a D.sub.c mixture, one must turn to one of the other processes that will be discussed, infra.
A second general route to D.sub.c is the liquid phase equilibration or "cracking" reaction in which mixtures of D.sub.c and/or linear polydiorganosiloxanes are reacted in the presence of a catalyst such as a base to form an equilibrium mixture of D.sub.c and linears. The D.sub.c is then removed batchwise or continuously from the equilibrium mixture by fractionation or other means of separation. In the liquid phase, the equilibrium mixture of D.sub.c to linears can be as low as 10 to 15 percent D.sub.c. Dilution with solvent to a siloxane content of approximately 20 to 25 percent can result in a D.sub.c content of the siloxane mixture of 70 to 75 percent. The liquid phase equilibrium and impact of solvent are discussed in Carmichael et al., J. Phys. Chem., 71(1967), pp. 2011-15.
Hunter et al., J. Am. Chem. Soc., 68(1946), pp. 667-672, describe a method for preparing and isolating polydimethylsiloxanes. Hyde, U.S. Pat. No. 2,438,478, issued Mar. 23, 1948, and Hyde, U.S. Pat. No. 2,455,999, issued Dec. 14, 1948, make similar disclosure. York, U.S. Pat. No. 2,816,124, issued Dec. 10, 1957, discloses a process for preparing hexaethylcyclotrisiloxane. Fletcher, U.S. Pat. No. 2,860,512, issued Nov. 11, 1958, discloses a method for preparing cyclic polydiorganosiloxanes in the presence of a high-boiling solvent. Gordon, U.S. Pat. No. 2,884,432, issued Apr. 28, 1959, discloses a process in which triorganosiloxanes are used to cause the contents of the equilibration vessel to remain fluid and to reduce the cracking temperature. Pierce and Holbrook, U.S. Pat. No. 2,979,519, issued Apr. 11, 1961, and Bluestein, U.S. Pat. No. 4,111,973, issued Sept. 5, 1978, disclose processes for the preparation of cyclopolytrifluoropropylmethylsiloxanes. Guinet and Puthet, U.S. Pat. No. 3,484,469, issued Dec. 19, 1969, and Kuznetsova et al., U.S. Pat. No. 3,558,681, issued Jan. 26, 1971, disclose processes for the preparation of cyclopolyphenylmethylsiloxanes. Macher, U.S. Pat. No. 3,607,898, issued Sept. 21, 1971, and Razzano, U.S. Pat. No. 3,846,464, issued Nov. 5, 1974, disclose processes for the preparation of cyclopolymethylvinylsiloxanes.
Okamoto and Yanagisawa, U.S. Pat. No. 3,989,733, issued Nov. 2, 1976, discloses a combination "cracking" and rectification process in a column-type reactor which uses as the column packing an alkaline catalyst in the form of pellets or an inert material upon which the alkaline catalyst is fused. In the continuous process disclosed, linear polydiorganosiloxanes and cyclopolydiorganosiloxanes are fed to the column. The raw material polysiloxanes fed to the catalyst zone undergo rearrangement in the course of flowing down through the catalyst zone, forming the desired cyclopolydiorganosiloxanes within the liquid layer on the catalyst surface, the cyclic siloxanes leaving as a vapor.
Hirakawa and Honda U.S. Pat. No. 4,197,251, issued Apr. 8, 1980, discloses a continuous process for producing octamethylcyclotetrasiloxanes by a cracking technique in which octamethylcyclotetrasiloxane is recovered and other cyclics species are recycled to the cracking reactor. Baile et al., U.S. Pat. No. 4,689,420, issued Aug. 25, 1987, discloses a two-step process in which water is removed from the polydiorganosiloxane feed before it is equilibrated to produce the desired cyclopolydiorganosiloxanes.
In a third route to D.sub.c, Reedy and Walsh, U.S. Pat. No. 4,556,726, issued Dec. 3, 1985, discloses a method for preparing decamethylcyclopentasiloxane, D.sub.5, from octamethylcyclotetrasiloxane, D.sub.4, by heating the D.sub.4 in the presence of aqueous hydrochloric acid and a salt of a protonated amine.
The instant invention involves the preparation and isolation of specific D.sub.c species via the vapor phase rearrangement of other D.sub.c materials or mixtures thereof. Rode, et al., Vysokomal.soyed.All, 11(1969), pp. 1733-1744, describes study on the thermal degradation and stabilization of polydimethylsiloxanes. Rode et al., studied the degradation of hydroxy-endblocked and trimethylsilyl-endblocked linear polydimethylsiloxanes. Rode et al., found that heating the polydimethylsiloxanes in an inert medium or under vacuum led to the formation of D.sub.c, mainly D.sub.3. However, Rode et al., found that in the presence of sodium hydroxide or sulfuric acid, no traceable D.sub.3 was present. Davidson and Thompson, Chemical Communications, (1971), pp. 251-252, discloses a study of the pyrolysis of octamethylcyclotetrasiloxane, D.sub.4, in the gas phase at temperatures of about 760.degree. to 842.degree. K. at pressures from about 0.5 to 13 mm Hg in a static system. Davidson and Thompson found that pyrolysis of up to about 25 percent of the D.sub.4 gave D.sub.5 and D.sub.6 as the only products. Gusel nikov et al., Bull. Acad. Sci., USSR, 20(1971), pp. 71-75, describes the pyrolysis of cyclodimethylsiloxanes --D.sub.3, D.sub.4, D.sub.5, and D.sub.6 --at 550.degree. C. for 3 hours. None of these references discloses the control of distribution of a cyclopolydiorganosiloxane mixture, by varying temperature and pressure, to prepare and isolate individual D.sub.c species.